1. Field of the Invention
The present invention relates to linear electric motors and more particularly to a high force, step and repeat piezoelectric linear motor.
2. Description of the Related Art
Many types of electrical motors have been developed that provide rotational force such as three-phase motors, induction motors, split-phase motors, etc. However, there is a need for motors that provide linear displacement for use in applications such as aircraft flaps, electrical powered sunroofs or electrically powered car seats. Some of the more heavy duty applications require a linear displacement force in excess of 220 Newtons.
Currently, ball and screw type motors are frequently used for producing linear displacement and comprise a threaded shaft and a metal ball with a threaded hole that mates with the threaded shaft. As the shaft turns, the metal ball rides up or down the shaft to linearly displace the metal ball and any attached mechanism. One problem with this motor is the high tolerance that is required between the ball and the shaft, particularly when used in high accuracy applications. Through use, the threads on the shaft and the ball tend to wear, resulting in a backlash when the motor is stopped or encounters a load, thus reducing positioning accuracy.
The ball and screw motor is powered by an electric motor which can overheat and be damaged or destroyed if the motor stalls under an excessive load. Also, electric motors generally work most efficiently at one speed; when the motor is slowed under load its efficiency drops.
Furthermore, the force produced by the shaft and ball displacement is directly related to the power of the electrical motor. For increased power, the size of the electrical motor must be increased. In applications requiring a great deal of linear displacement force, the size of the motor can become prohibitive. Ball and screw motors also tend to be relatively expensive.
Piezoelectric materials have been used for many different types of motors, primarily in motors that produce a rotation as opposed to displacement. As an example, see U.S. Pat. No. 5,780,956 to Oliver, Neurgaonkar, et.al. Certain piezoelectric materials are useful because of their ability to directly convert electrical energy into motion (mechanical energy). When a voltage is applied to the piezoelectric material, the material will experience a strain that causes it to expand. When the voltage is removed, the strain is removed and the material contracts.
Piezoelectric materials are generally formed from ceramics. One particularly valuable type of piezoelectric device has a plurality of laminated piezoelectric layers that can expand and contract quite rapidly, and combines the expansion of all the layers. The purpose of such layering is to keep the necessary drive voltage to a practical level, while obtaining significant expansion. The expansion can vary, but is generally on the order of 0.002 times the length of the layered piezoelectric material.
Linear piezoelectric motors have been developed using an "inchworm" piezoelectric mechanism to linearly translate a shaft. An example of this type of motor is the Burleigh PZ-577 Inchworm.TM. Translator System which comprises three piezoelectric cylinders coupled together on a shaft. One of the end cylinders is fixed to a support structure and the other cylinders are allowed to move linearly in relation to the fixed cylinder. The cylinders rely on an inchworm type motion to move the shaft. The first and third cylinders fit around the shaft with near zero clearance, while the middle cylinder has a clearance fit over the shaft. If the first cylinder were fixed, a voltage is applied to the first cylinder and it grips the shaft. A voltage is then applied to the middle cylinder causing it to expand longitudinally down the shaft, pushing the third cylinder ahead of it. A voltage is then applied to the third cylinder, causing it to grip the shaft. The voltage is next removed from the first cylinder causing it to release the shaft, and also from the middle cylinder, causing it to contract and pull the shaft with it. This inchworm cycle results in moving the shaft in the direction of the first cylinder and is repeated to move the shaft linearly.
The primary problem with inchworm type motors is that they typically provide linear push or pull forces in the range of 10 to 15 Newtons and cannot be used for heavy duty applications requiring a greater linear force. Such motors also require precise machining and are not easily adjusted for optimum performance